Single stage photomultiplier tube



sarily bulky, complex, and expensive.

3,%9,59l Patented Dec. 18, 19%.?

This invention relates to photomultiplier tubes and more particularly to such tubes in which a single stage of multiplication is employed.

In the photomultiplier tubes which are generally. em-

ployed, a series of electrodes or dynodes having surfaces .WhlCh readily generate secondary emission electrons, are

employed. Because of the number of dynodes in each photomultiplier tube (generally the tubes are neces- Furthermore, the required accurate alignment of they dynodes makes the tubes relatively sensitive to shock or high accelerations.

Accordingly, a principal object of the present invention is to simplify and reduce the cost of photomultiplier tubes, without sacrificing sensitivity.

Another object of the invention is to reduce the size and sensitivity to shock of photomultiplier tubes.

In accordance with one illustrative embodiment of the invention, a photomultiplier tube has a generally cylindrical configuration. The'outer envelope has a transparent -window with an inner coating of a photoemissive substance. A semiconductive body including a p-n junction is located at the opposite end of the tube, preferably on the axis of the cylindrical tube. A conducting shield extends along the curved walls of the tube, preferably into electrical contact with the photoemissive layer. A circular electrode may extend from the shield toward the semiconductive body to focus electrons from the photoemissive layer directly onto the body.

In accordance with a feature of the invention, therefore, a single stage photomultiplier includes an exposed photoemissive surface, a semiconductive body including a p-n junction spaced from the photoemissive surface, and an opaque conductive member for shielding the photoemissive layer from extraneous light and for focusing electrons onto the semiconductive body.

In accordance with additional features of the invention, the p-n junction has a high Zener breakdown voltage of the order of a thousand volts or more, and circuitry is provided for back biasing the p-n junction diode at a level below the Zener breakdown point and for providing a substantial accelerating voltage between the photoemissive layer and the semiconductor body.

The arrangements described above have the advantages of simplicity and economy. More specifically, the many dynodes required in a conventional photomultiplier are avoided. Furthermore, the simplified form of the tube lends itself readily to miniaturization and ruggedness of design.

Other objects, features and advantages of the present invention may be readily apprehended from a consideration of the following detailed description and from the drawings, in which,

FIG. 1 shows the structure of a single stage photomultiplier in accordance with the present invention;

FIG. 2 is an energization circuit for the arrangement of FIG. 1; and

FIG. 3 is a curve of conduction current plotted against beam current for a p-n junction such as that employed in FIG. 1.

With reference to the drawings, the principal components in the single stage photomultiplier of FIG. 1

glass or quartz, for example. The photoemissive layer 12 is on the inner surface of the transparent envelope. The liner 16 may be a metallic coating on the inner surface of the glass envelope 18 or may constitute the vacuumtight envelope around the side of the vacuum tube. The electrode 20 is in electrical contact with the conductive liner 16 and is apertured to permit the passage of electrons from the photoemissive layer 12 to the p-n junction 14. Suitable lead-in pins 22, 24 and 26 are provided to apply appropriate potentials to the conductive liner, focusing electrode and photoemissive layer, and across the p-n junction 14.

FIG. 2 shows a high voltage power supply 28, a voltage divider including resistors 3d and 32, the p-n junction 14 and an output resistor 34. The high voltage lead 36 of FIG. 2 is connected to terminal pin 26 of FIG. 1 to supply high negative voltage in the order of several kilovolts to the focusing electrode, the conductive liner and the photoemissive layer 12. Leads 33 and 4d are connected to terminal pins 22 and 24 to properly back bias the p-n junction 14. The output voltage developed across the load resistor 34 appears at terminal 42.

Now, considering the mode of operation of the single stage photomultiplier, the high potential established between the semiconductive body 14 and the layer 12, which is also in contact with liner 16 and the electrode 29, establishes an electric field which is generally indicated by the dashed lines 54 in FIG. 1. With the electric field taking this form, electrons generated at the surface of the photoemissive layer 12 are accelerated at high velocities into the semiconductor body 14.

The semiconductor junction 14- is formed by known semiconductor fabrication techniques so that it has a high Zener breakdown region at a level of several hundred or is now possible to obtain Zener breakdown levels at nearly any desired reverse voltage level from less than one volt, for example, up to several thousand volts. The p-n junction is oriented parallel to the surface of the semiconductor body, facing the electron beam. It may be formed by diffusing suitable impurities to provide a surface layer of one type of semiconductive material on a body of material of the other conductivity type. Basic techniques for forming p-n junction of various types are disclosed in the Bell System Technical Journal, vol. 35, pages 661684 (1956). In the present case, the low current capacity which is required simplifies the fabrication problem.

The amplificaiton obtained in the p-n junction diode is the product of two factors. The first factor involves the conversion of the energy of the electron beam into an electron-hole pair conduction current in the semiconductive body. This is enhanced by the acceleration and resultant avalanche effect of these current carriers in the high field region of the biased junction. This. effect is particularly pronounced when the semiconductor has a relatively high Zener breakdown voltage and is biase just below this breakdown level. In the present example, the Zener breakdown voltage of diode 14 is above 1000 volts, and a back voltage of about 1000 volts is applied across the junction.

The photoemissive layer 12 emits free electrons within the vacuum tube in response to incident light radiation. The spectral sensitivity of the photomultiplier depends on the composition of the photoemissive surface. A large number of these surfaces have been developed and standardized. The spectral response of a number of the surfaces are given in the Handbook of the American Institute of Physics, 1957 edition, section 6, pages 116 and 117. In accordance with a relatively recent development, a very'sensitive surface in the visible light range is a trialkaline surface containing a mixture of lithium, potasa sium and sodium. Other important surfaces include the infrared surface designated 8-1 in the handbook cited above and the 3-11 cesium-antimony surface which is most sensitive to blue radiation. Any of these known surfaces may be employed as the surface 12 in the arrangement of FIG. 1.

FIG. 3 shows the short-circuit conduction current which is produced by an input beam current of varying magnitude. Three difierent levels of acceleration are shown in the plot of FIG. 3; specifically, plot 46 shows conduction current versus beam current for an acceleration of the beam of 3000 volts. Plots 48 and 50 show comparable plots for acceleration voltages of six kilovolts and fourteen kilovolts, respectively. The ratio of the conduction current 1 to the beam current l is the multiplication factor obtained from bombardment. At 14 kilovolts this gain is just under 2,000. it is particularly to be noted that the plots of FIG. 3 merely represent short-circuit current and do not include the additional effect of amplification by the avalanche effect within the semiconductor structure. In the arrangement as described above, with a voltage of approximately 1 kilovolt applied across the p-n junction, an additional current amplification of approximately 400 is obtained. This corresponds to a total multiplication factor of approximately 7 800,000 in the output current flowing through the semiconductor 14 with respect to the variable beam current from the photoemissive layer 12. Higher multiplication can be obtained with the use of higher electron accelerating voltages and with the use of junctions with higher Zener breakdown voltages.

The general design of the photomultiplier tube as set forth above is also applicable to image dissector tubes. With reference to FIG. 1, certain modifications would be necessary for use as an image dissector. First, a relatively small p-n junction target would be employed. Secondly, the electrons from the photoemissive layer would be collimated by known techniques, instead of belng concentrated onto the semiconductor body as shown in FIG. 1. Sweep coils would also be provided around the tube to shift the collimated beam of electrons from the photoemissive layer with respect to the target junction. In this manner, an effective scanning raster would be established. The pattern, or picture, supplied to the surface of the photo-emissive layer 12 would then be dissected and would appear at the output from the p-n junction 14.

To restate some of the advantages of the present tube, A

they include simplicity, economy and compactness. Thus, for example, while conventional photomultiplier tubes are normally five or six inches long, the present tube may be less than one inch in length. In addition, the requtrement for precise alignment of the many dynodes required in conventional photomultiplier tubes means that they are relatively sensitive to shock, whereas the pres- 'ent simple construction can be readily ruggedized as voltage, conducting means for both shielding said layer and semiconductive body from extraneous radiations and for focusing electrons generated at said photoemissive layer onto said semiconductive body, means for applying an accelerating voltage between said layer and said body,

and means for back biasing saidjunction to a level close :to the Zener breakdown voltage. a

2. A single stage photomultiplier tube comprising a photoemissivc layer, a semiconductive body including a pn junction, and conductive means for shielding said layer and semiconductive body from extraneous radiations and means for applying a potential to said conductive means to focus electrons generated at said photoernissive layer onto said semiconductive body.

3. A single stage photomultiplier tube comprising a photoemissive layer, a semiconductive body including a p-n junction, conducting means for both shielding said layer and semiconductive body from extraneous radiations and for focusing electrons generated at said photoemissive layer onto said semiconductive body, and means for applying voltages to said body and said conducting means.

4. A generally cylindrical single stage photomultiplier tube comprising a photoemissive layer exposed to light at one end of said tube, a semiconductive body including a p-n junction spaced from said layer within said tube, conductive means substantially enclosing the space between said layer and said body for both shielding said layer and semiconductive body from extraneous radiations and for focusing electrons generated at said photoemissive layer onto said semiconductive body, and terminals connected to said conductive means and said body for applying focusing and back biasing voltages to said conductive means and said junction, respectively.

5. A single stage photomultiplier tube comprising a photoemissive layer, a semiconductive body including a p-n junction, a focusing electrode and means for applying a potential to said electrode to produce an electrical field for directing electrons generated at said photoernissive layer onto said semiconductive body.

6. A generally cylindrical single stage photomultiplier tube comprising a photoernissive layer exposed to light at one end of said tube, a semiconductive body includ ing a p-n junction spaced from said layer within said tube, conductive means substantially enclosing the space between said layer and said body, for both shielding said layer and semiconductive body from extraneous radiations. and for focusing electrons generated at said photoemissive layer onto said semiconductive body, and a conductive plate adjacent said body having an aperture for permitting the flow of electrons from said layer to said body, said conductive plate, said conductive means and said layer being in electrical contact.

l photoernissive layer, a semiconductive body including a 'p-n junction, conducting means for focusing electrons generated at said photoemissive layer onto said semiconductive body, said junction being oriented generally perpendicular to the path of incident electrons, and'means for applynig voltages to said body and said conducting means.

References Cited in the file of this patent UNITED STATES PATENTS 

